Possible Reasons, why your Texture is not showing -
- You've not UV Unwrapped your Model.
- There might not be enough Light in the scene to light up your model
- You've added wrong textures/values in the wrong Slot i.e. Alpha or Transparency.
- It might be a bug in Renderer.
Steps to Fix it -
1.Just Import your 3D Model in Blender. Go to Edit Mode. You'll see that your model becomes Opaque and Opaque Wireframes have become visible. Now, press "A" key from the keyboard. You'll see that the wireframes have got Yellow Color. Now press "U" Key from the Keyboard. Few Options might pop out. Select, "Smart UV Unwrap" or any similar Option from it. Now, you can further, proceed by saving that model again.
Use a Directional Light.
You've to first apply your texture in Albedo/Diffuse (Albedo probably in Godot v3.0), then you've to check if that Works or not, then You've to apply other textures.
If you think that it's not working as it was supposed to do, then you can just, write that issue, in Github and those problems will be fixed (most probably depending upon the degree of Errors in Coding) in the next Release of Godot i.e. v3.0.1 or v3.1.
Note- also note, that if your model is too big/complicated and the texture size is too small (128 x 128, 64 x 64, etc.) then you've to either, tile that image to get perfect details or you've to use a big texture instead.